DEVELOPMENT OF HOUSE HEATING 117 



the edge of the tub just in proportion as his body was im- 

 mersed, and also that his own weight was decreased at the 

 same time, he leaped from the bath and ran to his home 

 shouting, " Eureka, Eureka," which means "I have found 

 it, I have found it." Pure gold is a little more than nineteen 

 times as heavy as water, while silver is but ten and one-half 

 times as heavy. If the crown had been pure gold it should 

 have lost one-nineteenth of its weight when immersed in 

 water. He found that it lost more than one-nineteenth of 

 its weight in water, so he concluded that silver had been 

 used in its construction. 



ARCHIMEDES' PRINCIPLE may be thus stated: Whenever a 

 body is immersed in a fluid (liquid or gas) it is buoyed up with 

 a force which exactly equals the weight of the fluid displaced. 



When the cork was immersed in the water (d, Ex. 36), it 

 was being pushed upward with a force equal to the weight of 

 an equal volume of water. Since the weight of the cork was 

 less than this force, it was pushed to the surface of the water 

 and partly out of it. As it floated upon the surface of the 

 water, it was displacing an amount of water which exactly 

 equaled it in weight. The stone, on the other hand, sank to 

 the bottom because it was heavier than an equal volume of 

 water; its weight was therefore greater than the buoyancy of 

 the water. That it was buoyed up to a considerable extent 

 was shown by the balance. 



135. Convection Currents Caused by the Buoyant Effects 

 of the Air. The solid portion of the earth is covered by an 

 ocean of air many miles, probably some hundreds of miles, in 

 depth. This air has weight just as water has weight. In 

 fact, it is much heavier than we usually suspect until we have 

 weighed some of it. A box 2 ft. by 2 ft. by 3 ft., or 12 cu. ft., 

 holds 1 Ib. of air. A common schoolroom 30 ft. by 30 ft. by 

 10 ft. therefore holds 750 Ib. of air. 



All bodies here on the earth's surface are being buoyed up 

 by this air exactly as the stone and the cork were buoyed up 

 by the water. A stone will fall through the air and rest at 



